1. Field
This disclosure is generally related to a solar cell. More specifically, this disclosure is related to a solar cell that uses an aluminum grid as a backside conductor.
2. Related Art
The negative environmental impact caused by the use of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability.
A solar cell converts light into electricity using the photoelectric effect. There are several basic solar cell structures, including a single p-n junction, p-i-n/n-i-p, and multi-junction. A typical single p-n junction structure includes a p-type doped layer and an n-type doped layer. Solar cells with a single p-n junction can be homojunction solar cells or heterojunction solar cells. If both the p-doped and n-doped layers are made of similar materials (materials with equal bandgaps), the solar cell is called a homojunction solar cell. In contrast, a heterojunction solar cell includes at least two layers of materials of different bandgaps. A p-i-n/n-i-p structure includes a p-type doped layer, an n-type doped layer, and an intrinsic (undoped) semiconductor layer (the i-layer) sandwiched between the p-layer and the n-layer. A multi junction structure includes multiple single junction structures of different bandgaps stacked on top of one another.
In a solar cell, light is absorbed near the p-n junction, generating carriers. The carriers diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a solar cell's quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to electrical energy) and power collected when the solar cell is connected to an electrical circuit.
FIG. 1 presents a diagram illustrating an exemplary homojunction solar cell based on a crystalline-Si (c-Si) substrate (prior art). Solar cell 100 includes a front side electrode grid 102, an anti-reflective SiN layer 104, an n+ c-Si emitter layer 106, a p-type c-Si substrate 108, and an aluminum (Al) backside electrode 110. Arrows in FIG. 1 indicate incident sunlight. Note that when forming Al backside electrode 110, standard operations include screen-printing and firing of Al paste. Al forms a eutectic alloy with Si at a temperature of 577° C. During the firing process, a liquid Al—Si phase is formed according to the Al—Si phase diagram. The molten Al—Si region acts as a sink for many impurities, giving a perfect gettering effect. The p+ region generated by the firing of Al forms a back surface field (BSF), which introduce a barrier to minority carrier flow to the back surface of the solar cell. Note that the whole backside of the solar cell needs to be covered by the Al paste to ensure sufficient passivation.
Based on industrial surveys, crystalline-Si-wafer based solar cells dominate nearly 90% of the market. However, the cost of conventional solar grade Si is well above $100/kg, which drives the cost of solar cells to $3-$4 per Watt peak (Wp). In addition to the cost of solar grade Si wafers, the cost of Al used for the backside electrode can also be significant, given that a large amount of Al is needed to cover the whole backside of the solar cell.